Interpretive Summary: Stem rust is one of the most serious diseases of wheat, barley, and oat. Rust epidemics can cover wide regions of the U.S., causing substantial losses in yield and quality of these grains. Effective resistance to rust is essential for economic production of wheat throughout the Great Plains in most years. Wheat yields have been reduced by as much as 50% by stem rust in Minnesota and North Dakota in years when the resistance was not effective. Populations of rust fungi often contain many different pathogenic races, and these races can change over time. This makes it difficult for plant breeders to develop varieties that will remain resistant to all races of rust that might attack them. We studied competition between races of wheat stem rust to learn more about how rust races affect each other in mixed populations and to better understand what factors determine why some races become dominant in mixed populations. We found that competing stem rust races respond to the presence of other race on the same wheat leaves by accelerating their rates of spore production. This previously unrecognized effect of competition in rust fungi may be important in determining how fast rust epidemics develop. Understanding this effect of competition should also help plant pathologists determine what makes some races more successful competitors than others. By identifying which rust races are most competitive and aggressive, plant pathologists can set priorities for plant breeders in choosing new types of rust resistance directed against the most dangerous races. Prioritizing breeding efforts will avoid serious losses to rust epidemics and optimize use of breeding resources.

Technical Abstract:
To investigate the effects of competition on the timing of pathogen reproduction, urediniospores of two strains of Puccinia graminis f.sp. tritici were inoculated onto leaves of wheat seedlings singly and in 1:1 mixture at three inoculum densities. On randomly sampled leaves, uredinia were counted 9 days after inoculation and urediniospores were collected and dquantified every other day from the 7th through 29th day after inoculation Increases in inoculum density resulted in progressively smaller increases in uredinial numbers. However, total urediniospore production per leaf was not significantly affected by inoculum (and hence uredinial) density on the leaf over a range from 10 to 300 uredinia per leaf. Total urediniospore production per pustule generally decreased with increasing density. At high density, sporulation per pustule attained a maximum level earlier in the sporulation period, had a less distinct peak, and dropped off earlier than for the lower densities. Logistic model fits to cumulative sporulation curves over time revealed that strain SR41 had a greater epidemic rate parameter (r) than strain SR22 at low and intermediate densities, while SR22 had a higher r-value than SR41 at high density. Strains also exhibited greater r-values in the presence of the other strain than when alone. Results suggest that strains may have different evolutionary strategies in their timing of reproduction, and that both intra- and inter-strain competition can have complex effects on the temporal dynamics of sporulation in pathogen strains.